Silicon-based electronics systems have become increasingly important in recent years, especially for automotive applications. These silicon-based electronics are used principally for storing control algorithms, process information and for directing actuators to perform various functions, including steering, suspension and display of driver information, to name but a few. While the design of electronics has advanced rapidly, the development of sensor technology has not proceeded at the same rate, and sensor designs continue to be based on dated technologies which have inbred limitations. Silicon has recently been identified as the basis for future sensor technology, and this hopefully will close the technology gap and permit greater application of control systems utilizing sensor technology.
Silicon is now widely recognized in the industry as being suitable for use in silicon-based electronics, and silicon sensor designs can now be created using a variety of manufacturing processes, one of the most promising of which is referred to as "micromachining" which uses chemical processes to introduce three-dimensional mechanical structures into silicon. These "microstructures", as they are referred to, can be made sensitive to specific physical phenomena, such as acceleration, pressure and fluid flow, so that it is possible to fabricate accelerometers, pressure sensors and mass air flow sensors (MAFS), including hot wire anenometers and fuel flow rate detectors. Different aspects of micromachining are reviewed in Lee et al, "Silicon Micromachining Technology for Automotive Applications", SAE Publication No. SP655, Feb. 1986, and the content of that publication is hereby incorporated by reference.
In order to improve the performance of such devices, it is important for the heater/sensor element to have a substantially constant and preferably highly linear temperature coefficient of resistance, which does not change with thermal ageing. In the past, gold has been used as the heater/sensor element but this has not met with acceptance due to the fact that gold is not compatible with most semiconductor processes, and has a low resistivity, thereby requiring a long resistor which uses valuable real estate on the silicon wafer. Attempts have been made to improve the metallization characteristics of gold when used in conjunction with semiconductors by using a chromium/gold metallization system, but this too has proved unsuccessful because of interdiffusion characteristics at temperatures higher than about 200.degree. C. Since mass air flow sensors are usually operated at temperatures of at least 200.degree. C., the material used for sensing and heating elements in such sensors must have stable electric characteristics under those heat conditions and, in particular, must exhibit a stable thermal coefficient of resistance and sheet resistivitY (R-sh).